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Skip to 0 minutes and 6 seconds[Radioactivity in smoke alarms] We all have, or should have, smoke detectors in our homes. Smoke alarms contain a small amount of the radioactive element, Americium-241. Americium is an alpha and gamma emitter. Inside the smoke detector, the alpha particles pass through an ionisation chamber, creating a small current between two electrodes. When smoke particles block some of the alpha particles, the current drops, causing the alarm to sound.

Skip to 0 minutes and 46 secondsIn the classroom, we can remove the plastic casing of the smoke alarm. Here we see the aluminium shielding around the source. The shielding should never be removed to expose the source. Any alpha particles will be blocked by this shield and so pose no danger to us. Gamma radiation will, however, penetrate the shield and levels can be measured with a Geiger-Muller tube. It is important that background levels of radiation are measured first before taking a measurement next to the smoke detector so that students can calculate the count rate from the Americium source. Since gamma rays are just high energy photons of light, their energy drops off with the square of the distance away from the source.

Skip to 1 minute and 30 secondsEven though radiation levels above background can be detected with the Geiger-Muller tube next to the source, students should be able to appreciate that when placed on the ceiling, radiation levels at head height directly under the source will be much lower. In fact, the dose levels of radiation over one year, from smoke detectors in the home have been calculated to be less than that from the potassium contained in one banana.

Smoke alarms

This video discusses how smoke alarms use a radioactive source to detect smoke particles, what the safety implications are for their use at home and as a source in the laboratory. The video explains that gamma radiation is electromagnetic radiation, and thus it is made of photons - just like visible light but higher energy.

Smoke alarms provide a simple way of bringing context to the topic of radioactivity. Despite the shielding on the source blocking the alpha radiation, we can use the Americium as a weak gamma source. The background count and the count rate with the source should be measured over a period of at least 5 minutes to ensure a measurable count above background.

This could be an opportunity for students to investigate the question ‘Are radioactive smoke detectors dangerous to our health?’. They could investigate this by using their knowledge of how alpha particles are blocked by a sheet of paper and by plotting graphs of how gamma radiation drops off with distance.

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This video is from the free online course:

Teaching Practical Science: Physics

National STEM Learning Centre